The present invention relates in general to monitoring devices for detecting abnormal conditions in a circuit consuming electrical power, and more particularly to a load current monitor for detecting a change in load current that indicates an abnormal variance in load impedance.
A monitoring device in accordance with the present invention is especially useful for monitoring the status of a heat trace cable constituted by a large plurality of parallel connected resistance wire heating elements physically positioned serially along the length of the cable.
One or more heat trace cables are often wrapped around lengths of exposed chemical process piping to maintain the piping at a predetermined temperature. It is important to monitor the status of the cable in a manner that will determine when one or more of the many parallel heating elements burns out, i.e., open circuits, thus raising the overall impedance of the cable, thereby lowering its heating ability.
It is known in the art to utilize a current transformer to monitor only the load current flowing into a heat trace cable. Over a period of time, as individual heating elements in the cable burn out and fail, the overall cable impedance gradually rises to decrease the load current, the current transformer providing to associated cable control circuitry a signal indicating the current decrease. At a predetermind degree of load current decrease, the control circuitry indicates an "underheating" alarm condition for the monitored cable. The defective cable can then be replaced with a new one, or other corrective action may be taken.
The sensitivity of such a heat trace cable "current only" monitoring technique must necessarily be limited to account for load voltage fluctuations induced by commercially supplied line voltage used to directly power the heat trace cable. Commercially supplied line voltage often varies and fluctuates over a normal range, as is well known in the art. A decrease in the supplied voltage (commercial line voltage) will cause a decrease in load current for a constant impedance, non-defective, heat trace cable. This load voltage, fluctuation-caused current decrease does not indicate a cable failure (i.e., underheating). Thus, the sensitivity of the above-described load current monitor is limited to avoid false indications of cable failure for normal load voltage fluctuations.
It is desirable to provide a load current monitor for a heat trace cable, or other generally constant impedance load that disregards load current changes caused by fluctuations in applied load voltage, e.g., commercial line voltage.
It would also be desirable to provide alarm circuitry working in conjunction with a plurality of current-monitored heat trace cables, such alarm circuitry being, at least in part, resettable without having to correct one or more faulty cable of the many being monitored by use of the common alarm circuitry.